In the context of production from a well, oil and gas are understood to refer to crude oil and natural gas. Oil and gas are naturally occurring hydrocarbons in certain subterranean formations.
A subterranean formation is a body of rock that has sufficiently distinctive characteristics and is sufficiently continuous for geologists to describe, map, and name it. A subterranean formation having a sufficient porosity and permeability to store and transmit fluids is sometimes referred to as a reservoir. A subterranean formation containing oil or gas may be located under land or under the seabed off shore. Oil and gas reservoirs are typically located in the range of a few hundred feet (shallow reservoirs) to a few tens of thousands of feet (ultra-deep reservoirs) below the surface of the land or seabed.
To produce oil or gas, a well is drilled into a subterranean formation that is an oil or gas reservoir. A well includes a wellhead and at least one wellbore from the wellhead penetrating the earth.
Typically, a wellbore must be drilled hundreds or thousands of feet into the earth to reach an oil or gas bearing formation. Generally, the greater the depth of the formation the higher the static temperature and pressure of the formation.
Generally, well services include a wide variety of operations that may be performed in oil, gas, geothermal, or water wells, such as drilling, cementing, completion, and intervention. These well services are designed to facilitate or enhance the production of desirable fluids such as oil or gas from or through a subterranean formation.
In general, drilling is the process of drilling the wellbore. After the hole is drilled, sections of steel pipe, referred to as casing, which are slightly smaller in diameter than the borehole, are placed in at least the uppermost portions of the wellbore. The casing provides structural integrity to the newly drilled borehole.
Cementing is a common well operation. For example, cement compositions can be used in cementing operations in which a string of pipe, such as casing or liner, is cemented in a wellbore. After setting, the cement stabilizes the pipe in the wellbore and prevents undesirable migration of fluids along the annulus between the wellbore and the outside of the casing or liner between various zones of subterranean formations penetrated by the wellbore. Where the wellbore penetrates into a hydrocarbon-bearing zone of a subterranean formation, the casing can later be perforated to allow fluid communication between the zone and the wellbore. The cemented casing also enables subsequent or remedial separation or isolation of one or more production zones of the wellbore, for example, by using downhole tools such as packers or plugs, or by using other techniques, such as forming sand plugs or placing cement in the perforations. Cement compositions can also be utilized in intervention operations, such as in plugging highly permeable zones or fractures in zones that may be producing too much water, plugging cracks or holes in pipe strings, and the like.
After drilling and cementing the casing, completion is the process of making a well ready for production or injection. This principally involves preparing a zone of the wellbore to the required specifications, running in the production tubing and associated downhole equipment, as well as perforating and stimulating as required.
Intervention is any operation carried out on a well during or at the end of its productive life that alters the state of the well or well geometry, provides well diagnostics, or manages the production of the well. Workover can broadly refer to any kind of well intervention that involves invasive techniques, such as wireline, coiled tubing, or snubbing. More specifically, though, workover refers to the process of pulling and replacing a completion.
A well service usually involves introducing a well fluid into a well. As used herein, a “well fluid” is a fluid used in a well service. As used herein, a “well fluid” broadly refers to any fluid adapted to be introduced into a well for any purpose. A well fluid can be, for example, a drilling fluid, a cement composition, a treatment fluid, or a spacer fluid. If a well fluid is to be used in a relatively small volume, for example less than about 200 barrels (32 m3), it is sometimes referred to in the art as a wash, dump, slug, or pill.
Hydraulic cement is a material that when mixed with water hardens or sets over time because of a chemical reaction with the water. The cement composition sets by a hydration process, and it passes through a gel phase to solid phase. Because this is a chemical reaction with the water, hydraulic cement is capable of setting even under water. The hydraulic cement, water, and any other components are mixed to form a cement composition in the initial state of slurry, which should be a fluid for a sufficient time before setting for pumping the composition into the wellbore and for placement in a desired downhole location in the well.
In performing cementing, a cement composition is pumped as a fluid (typically in the form of suspension or slurry) into a desired location in the wellbore. For example, in cementing a casing or liner, the cement composition is pumped into the annular space between the exterior surfaces of a pipe string and the borehole (that is, the wall of the wellbore). The cement composition is allowed time to set in the annular space, thereby forming an annular sheath of hardened, substantially impermeable cement. The hardened cement supports and positions the pipe string in the wellbore and fills the annular space between the exterior surfaces of the pipe string and the borehole of the wellbore.
It is important to maintain a cement in a pumpable slurry state until it placed in a desired portion of the well. For this purpose, a cement retarder, which is sometimes referred to as a set retarder or simply a retarder, can be used in a cement composition. A retarder retards the setting process and helps provide adequate pumping time to place the cement slurry.
Without being limited by any theory, it is believed a retarder works by one or more of the principles of chelation, adsorption, or precipitation.
In general, the selection of a cement retarder depends upon the well temperature. In addition, different thickening time can be achieved at particular temperature by varying the concentration of the retarder in the cement composition. Some of the known retarders work at a low temperature range while others work at high temperature range.
Phosphonate retarders are known to work at high temperature (450° F. to 550° F.) as described in CA1258366. Borates (e.g., sodium pentaborate and potassium pentaborate) and organic acids (e.g., citric acid and tartaric acid) are used as retarder or intensifier for high temperature. Similarly, polymeric retarder containing phosphate groups has been described in GB2443923 to work at temperature 300° F. to 600° F. These retarders are not desirable for low temperature application, however, because they are too sensitive to concentration. A slight inadvertent change in concentration during field operation may adversely affect the thickening time. It is desirable to have a retarder which performs well at low as well as high temperature, including by not being too sensitive to concentration at the design temperature.
It would be desirable to have a single polymer that could be used to help control the thickening time of a cement composition over a wide range of temperatures and without being too sensitive to concentration in the cement composition over the wide range of temperatures.